1. Field of the Invention
The present invention relates to a method of shutting off a fuel gas manufacturing apparatus for manufacturing a hydrogen-rich fuel gas by modifying a hydrogen-containing fuel which contains hydrocarbon or alcohol.
2. Description of the Related Art
There has been employed in the art a hydrogen manufacturing apparatus (fuel gas manufacturing apparatus) for modifying a hydrocarbon fuel such as natural gas or the like or a hydrogen-containing fuel containing alcohol such as methanol or the like to produce a hydrogen-containing gas (modified gas) and supplying the hydrogen-containing gas as a fuel gas to a fuel cell or the like.
For example, Japanese Laid-Open Patent Publication No. 2002-20102 discloses a hydrogen manufacturing apparatus basically having, as shown in FIG. 8 of the accompanying drawings, a hydrogenerated desulfurizer 2 for being supplied with a fuel such as a city gas or the like from a compressor 1, a water-vapor modifier 3 for modifying a desulfurized fuel with water vapor to produce a high-concentration hydrogen-containing gas (hydrogen-rich gas), a catalytic combustor 4 disposed around the water-vapor modifier 3 for burning hydrogen with oxygen in air in the presence of a catalyst, a gas modifier 5 for converting carbon monoxide contained in the hydrogen-containing gas into carbon dioxide and hydrogen, and a PSA (Pressure Swing Adsorption) device 6 for separating high-purity hydrogen from the hydrogen-containing gas which has been modified by the gas modifier 5 according to pressure adsorption.
The PSA device 6 is connected to a hydrogen storage tank 8 for temporarily storing the high-purity hydrogen before it is supplied to a polymer electrolyte fuel cell (PEFC) 7, and an off-gas holder (off-gas tank) 9 for temporarily storing an off gas (unwanted materials) adsorbed from the high-purity hydrogen by the PSA device 6. The off-gas holder 9 supplies the off gas as a fuel for heating the water-vapor modifier 3 to the catalytic combustor 4.
The PSA device 6 has a plurality of adsorption towers each filled with an adsorbent for selectively adsorbing components other than hydrogen under high pressure and desorbing the adsorbed components under reduced pressure. Each of the adsorption towers cyclically operates in adsorption, desorption, replacement, and pressurization processes for extracting high-purity hydrogen and discharging other components as an off gas.
Depending on the time at which the hydrogen manufacturing apparatus is shut off, the towers of the PSA device 6 tend to be shut off at different operative positions. If each of the towers is not shut off at a proper operative position, then an abnormal residual pressure may possibly remain in the tower, and an off gas may remain under an abnormal pressure in an off-gas passage. Furthermore, the hydrogen-rich gas is liable to remain in the water-vapor modifier 3 and the passage extending from the water-vapor modifier 3 to the PSA device 6.
When the hydrogen manufacturing apparatus starts operating normally after it is shut off abnormally, an off gas initially discharged from the towers and an off gas in the off-gas passage tend to be excessively calorific with respect to the capacity of the catalytic combustor 4, possibly damaging the catalytic combustor 4. In addition, a hydrogen gas of low purity may be stored in the hydrogen storage tank 8.